Liquid-crystal displays (LCDs) can achieve low-power consumption and are small and thin when compared with cathode-ray tubes (CRTs). Various sized LCDs are widely used in small apparatuses, such as mobile phones, digital cameras, personal digital assistants (PDAs), and large-sized LCD television sets.
Liquid-crystal displays are categorized into a transmissive type, a reflective type, and the like. In particular, transmissive liquid-crystal displays each include a liquid-crystal panel in which a liquid-crystal layer is disposed between a pair of transparent substrates and a backlight unit. Furthermore, a structure in which functional sheets, such as a diffusing sheet and a prismatic sheet, for irradiating the entire surface of the liquid-crystal panel with light from the backlight unit as a light source are disposed between the liquid-crystal panel and the backlight is generally used. Moreover, a structure in which a polarizer for polarizing light incident on the liquid-crystal layer and a color filter for displaying a color image are disposed on the liquid-crystal panel is generally known.
Various modes, such as a twisted nematic (TN) mode, a vertical alignment (VA) mode, and an in-plane switching (IPS) mode, are known as displaying methods for liquid-crystal displays. In any of these methods, the orientation of liquid-crystal molecules is changed in each pixel to control polarization or transmittance for light incident on the liquid-crystal layer, thereby displaying an intended image on the front surface of the liquid-crystal panel.
However, the polarization state of light coming through the liquid-crystal panel significantly depends on the orientation angle of the liquid-crystal molecules. Hence, the polarization state of light that is perpendicularly incident on the liquid-crystal panel different from the polarization state of light that is obliquely incident on the liquid-crystal panel, thus resulting in the difference in transmittance. Furthermore, retardation represented by the product of the refractive index anisotropy and the thickness of the liquid-crystal layer exhibits wavelength-dependent dispersion. The transmittance of light varies in response to a wavelength. As a result, the intensity of a light component having a specific wavelength range varies in response to the direction from which the liquid-crystal panel is viewed, thus changing the color temperature of an image. Furthermore, viewing-angle properties are disadvantageously degraded as follows: for example, display contrast and display colors vary in response to a viewing direction (see Japanese Unexamined Patent Application Publication No. 10-282498). In general, the intensity of blue often decreases and the intensity of red often increases, as a viewing direction becomes more oblique. Thus, the color temperature of an image tends to decrease.
To overcome the problems with the viewing-angle properties, in the known liquid-crystal display, a compensation film, such as a retardation plate or a color compensation plate, is disposed in a liquid-crystal panel to cancel the birefringence of a liquid-crystal layer, thereby improving the viewing-angle properties of the liquid-crystal panel (for example, see Japanese Unexamined Patent Application Publication Nos. 8-15695 and 11-24066). Furthermore, in some cases, a color-compensation function is imparted to a polarization component separating element for extracting a predetermined polarization component alone (see Japanese Unexamined Patent Application Publication No. 2004-309618).
However, the viewing-angle properties of the liquid-crystal panel are markedly affected by not only the birefringence of the liquid-crystal layer but also optical properties of a polarization plate and a color filter disposed in the liquid-crystal panel. A structure and combination of other constitutional elements other than the liquid-crystal panel, for example, luminance-improving films such as a diffusing sheet, a prismatic sheet, and a polarization component separating element result in different viewing-angle properties.
Thus, viewing-angle compensation focused on the birefringence of the liquid-crystal layer alone has limitations. The optical design of a compensation film that can compensate the effect of constitutional elements other than the liquid-crystal layer has increased complexity and difficulty, resulting in insufficient productivity.
In order to enhance the reproducibility of display colors, LED (light-emitting diode) backlights each including light-emitting diodes of RGB three primary colors as a light source and wide-color-gamut cold cathode fluorescent lamp (CCFL) backlights including improved phosphors are used in liquid-crystal displays. In this case, the emission spectrum of red of the light source shifts to longer wavelengths, thus causing significant reddening when the liquid-crystal panel is viewed in an oblique direction.
It is therefore desirable to provide an optical element that inhibits a change in the chromaticity of an image area when a liquid-crystal panel is viewed in an oblique direction; a liquid-crystal panel; and a liquid-crystal display.